The present invention relates to an electrostatic recording system and, more particularly, to a multistylus type electrostatic recording head which is suitable for recording of image signals for a facsimile, printer or the like, a method for manufacturing the same, and an apparatus for practicing this method.
Various multistylus type electrostatic recording heads are known for electronically scanning image signals to be recorded. An electrostatic recording head for a facsimile is generally of a multistylus type wherein a number of recording electrodes which hold one-to-one correspondence with all the picture elements corresponding to one scanning line are arranged in an array in an insulator such as an epoxy resin mold. Auxiliary electrodes arranged on the front or back surface side of the recording medium closely contacted with the recording head help to perform electronic scanning of image signals of a recording medium.
FIG. 1 is a schematic perspective view of a conventional electrostatic recording head of the single surface control type wherein recording electrodes and auxiliary electrodes are arranged on the same surface with respect to the recording medium. A number of recording electrodes 10 are independently embedded at a pitch of several electrodes per milimeter in an electrode retainer 12 of an insulator such as an epoxy resin. Front ends of the recording electrodes 10 are exposed to the surface of the retainer 12. Auxiliary electrodes 14 are also embedded in the electrode retainer 12 at both sides of the recording electrodes 10 and on the same plane as the recording electrodes 10. The auxiliary electrodes 14 are also exposed to the surface of the retainer 12. The auxiliary electrodes 14 divide the recording electrodes 10 into groups of equal numbers and are arranged in correspondence with the respective groups. The auxiliary electrodes 14 serve to control the recording positions of the recording electrodes 10 and are thus also called control electrodes. Electrostatic recording is performed by dividing a recording voltage into two voltages which are applied to the recording electrodes 10 and the control electrodes 14. Then, recording is performed only at positions on an electrostatic recording paper sheet 16 where both voltages are applied simultaneously.
The recording mechanism of the electrostatic recording head of the single surface type of the electrode arrangement as described above will now be described with reference to FIG. 2. Referring to FIG. 2, a number of recording electrodes 10 are divided into a number of electrode groups each consisting of m recording electrodes 10. Then, the recording electrodes at the same positions in every other group are connected to each other to provide two systems GA and GB of electrode groups. The recording electrodes of each electrode group are arranged to extend from the center of the corresponding control electrode 14 to the center of the adjacent control electrode 14. With the electrode arrangement shown in FIG. 2, if the numbers of the recording electrodes 10 and the control electrodes 14 are respectively N and M, the following relation is held: EQU M=(N/m)+1
Electrostatic recording is simultaneously performed with m recording electrodes of each group. The control electrodes 14 are used to select the electrode groups for recording. More specifically, referring to FIG. 2, in order to record with a first electrode group G-1, 0 V and a high voltage such as -300 V which alone may not effect formation of an electrostatic latent image are applied, in accordance with the pattern to be recorded, to the system GA of the electrode groups. Simultaneously, a voltage which has substantially the same absolute value as the above-mentioned voltage and which has the opposite polarity thereto, for example +300 V, is applied to the first and second control electrodes of a system GC of the control electrodes 14. Then, a potential difference of 600 V or 300 V is established between the recording electrodes of the first electrode group G-1 and the electrostatic recording paper sheet (not shown). An electrostatic latent image is formed on the surface of the electrostatic recording paper sheet immediately below the recording electrodes at which the potential difference of 600 V is established. Next, the system GB of the recording electrodes is driven while applying a voltage to the second and third control electrodes. Thus, recording with the second electrode group G-2 is performed. Thereafter, scanning and recording of one line is performed by applying a voltage to two control electrodes while shifting one control electrode at a time and while alternately driving the system GA and GB of electrode groups.
In this manner, the recording electrode groups are divided into two systems and are alternately driven. Two control electrodes are simultaneously driven. This is to prevent a drop in the potential distribution of the electrostatic recording paper sheet 16 at the edges of the control electrodes 14 as indicated by the dotted lines in FIG. 3. With this arrangement, a substantially flat potential distribution as indicated by a solid line is obtained even at the edges of the control electrode 14. Then, an electrostatic latent image which is uniform in the scanning direction can be formed. In the arrangement shown in FIG. 1, the control electrodes 14 are arranged at both sides and in the vicinities of the recording electrodes 10. This is also for the purpose of obtaining a flat potential distribution of the electrostatic recording paper sheet 16 at the positions of the recording electrodes 10. Referring to FIG. 1, the control electrodes 14 at both sides of one recording electrode 10 are commonly connected.
FIG. 4 schematically shows the structure of a conventional electrostatic recording head having the electrode arrangement shown in FIG. 2. Referring to FIG. 4, electrode wires 18 corresponding to the recording electrodes shown in FIGS. 1 and 2 comprise, for example, nickel wires having a diameter of 0.04 to 0.1 mm and covered with a polyurethane layer. One ends each of the electrode wires 18 are embedded in a rod-shaped electrode retainer 12 of an insulator such as an epoxy resin. One ends of these electrode wires 18 are exposed to the surface of the retainer 12 in the longitudinal direction thereof to constitute the recording electrodes 10. The electrode wires 18 correspondent to each recording electrode 10 are divided into electrode groups each consisting of m electrodes. The electrode wires 18 at the same positions in each electrode group are commonly connected to define electrode wire sets 20 numbering 2m. Each electrode wire set 20 belongs to either of the systems GA and GB of the recording electrode groups. The electrode wire sets 20 are connected to the upper portion of a printed circuit board 22 to be connected to a recording electrode drive circuit (not shown) through connectors 24. High voltage pulses are thus applied to the electrode wire sets 20 in accordance with the image signals to be recorded.
Meanwhile, the control electrodes 14 of rectangular shape are embedded in the electrode retainer 12 in a similar manner to that of the recording electrodes 10. One ends each of the control electrodes 14 are arranged close to both sides of the associated recording electrodes and within the same plane of the recording electrodes. The control electrodes 14 are connected to the front and back surfaces of the printed circuit board 22 through lead wires 26, respectively. The control electrodes 14 of each pair opposing each other with the associated recording electrode therebetween are commonly connected by through holes. The control electrodes 14 are connected to a control electrode drive circuit (not shown) through the connectors 24 of the printed circuit board 22. Thus, high voltage pulses of opposite polarity to that of a voltage applied to the recording electrodes are applied to these control electrodes so as to control the recording position.
Although not shown in the figure, the electrode retainer 12 and the printed circuit board 22 are fixed by a casing to constitute an electrostatic recording head unit.
With a conventional electrostatic recording head of the configuration as described above, wiring of the recording electrodes or electrode wires is extremely complex. Specifically, the electrode wires 18 constituting the recording electrodes 10 shown in FIG. 4 must be grouped and those at the same positions of each group must be commonly connected to form the electrode wire sets 20. During such operation, it is extremely difficult to correctly group a great number of thin electrode wires according to their positions and orders. Moreover, connection of the electrode wires during wiring of the electrode wire sets is not easy either. Manufacture of such an electrostatic recording head thus requires much skill, and inspection and correction of erroneous connection is also extremely difficult.
The electrode wires 18 are arranged in a very complex manner and are in contact with each other after formation of the electrode wire sets 20. Although these electrode wires 18 are insulated, relatively high voltages applied to them can invite dielectric breakdown. Impact or vibration can also cause damage to them. The conventional electrostatic recording head as described above thus has problems of stability and reliability.
Another electrostatic recording head is also known which utilizes a printed circuit board of a different configuration. This printed circuit board is obtained by forming by photoetching on a copper-plated laminate, recording electrodes which are formed by photoetching, and a matrix circuit for forming electrode sets by commonly connecting the recording electrodes at the same positions of a number of electrode groups. With this arrangement, a highly advanced technique is required in order to manufacture recording electrodes which are arranged at a high pitch. The manufacturing yield of such heads is low and manufacturing cost is high. Since the recording electrodes are formed by photoetching a copper-plated laminate, the recording electrodes become smaller than those formed from electrode wires of the prior art as described above. The recording electrodes also become asymmetrical with respect to the center (for example, trapezoidal). When such recording electrodes are used for facsimile or the like, recording reliability and density are degraded.
As has been described above, in the conventional head shown in FIG. 1, the control electrodes 14 are embedded and exposed through the electrode retainer 12 together with the recording electrodes 10. These control electrodes 14 are manufactured in the manner as shown in FIG. 5. Specifically, a square bar 28 is milled to form teeth 30. The lead wires 26 are connected to these teeth 30 to form a control electrode element. A pair of control electrode elements is prepared. This pair of control electrode elements is integrally formed with a resin with electrode wires or recording electrodes therebetween. Thereafter, the integral body obtained is cut at dotted lines in FIG. 5.
With such a manufacturing method, milling takes time and increases the number of manufacturing steps, so that the manufacturing cost is increased.